For some, happiness is getting enough sleep. But for a small marsupial in Australia, sex is a higher priority than getting some shut-eye — and it is willing to sacrifice hours of it daily to reach its happy ending.
Scientists based in Australia have found that mouse-sized male antechinus trade sleep to leave more time for reproductive activities during mating season, with one male who was monitored halving his sleep time during that period.
The study, published Thursday in the journal Current Biology, is the first to show direct evidence for this type of “extreme” sleep restriction in any land-dwelling mammal, according to the researchers.
“Animals need to reproduce to pass on their genes, but they also need to sleep to survive,” said Erika Zaid, lead author and animal behavior researcher at La Trobe University in Melbourne.
“Animals that are long-lived like humans and elephants don’t have this pressure to reproduce in a short period of time. They have the luxury of being able to sleep as long (as) they want (and) need each day,” she said.
Males antechinus, unlike their female counterparts, are semelparous, which means they can only breed once during their lifetime, making lengthy sleep a luxury that could cost them their opportunity to pass on their genes, Zaid explained. Non-breeding dusky antechinus spend an average of 15.3 hours of the day asleep, according to the researchers.
“Sleep restriction in breeding male antechinus is likely to be an adaptive behavioral response driven by strong sexual selection,” the paper said. This drives them to compete with other males to reproduce with as many females as possible, before dying shortly after their first — and last — mating season.
To study the semelparous marsupials, researchers examined two antechinus species: dusky antechinus (Antechinus swainsonii) and wild agile antechinus (Antechinus agilis) both captive and wild.
Researchers found that males from both species were not only more active during mating season, but also slept less during the same period.
Data showed that males were sleeping three hours less per night, every night, for three weeks — approximately the length of the mating period. Males, which only live for 11 months, reproduce once in their lifetime before dying while females can reproduce more than once, Zaid said.
Sleep is “an essential and seemingly universal behavior in the animal kingdom,” said John Lesku, associate professor of zoology at La Trobe University and a sleep scientist, who was involved in the study.
“Sleeping three hours less per night impacts waking performance in humans, (while) antechinus did this for three weeks. Therefore, antechinus may be resilient to sleep loss and have an unknown mechanism to thrive on less sleep during this time, or they may accept the physiological costs of staying awake to secure paternity before they die,” Lesku said.
The paper suggests that sleep reductions were due to the reproductive pressures on the males during their only breeding season, with increased sexual activity positively related to increases in testosterone, the male sex hormone, during the same length of time.
Using accelerometers — instruments used to measure the acceleration of a moving body — the researchers tracked the movement of 15 dusky antechinus (10 male) before and during mating season.
Researchers took blood samples to measure any changes in hormones and took electrophysiological recordings from four males to measure how much they were sleeping.
Blood samples were also taken from 38 wild agile antechinus (20 male) to see if oxalic acid, a biomarker for sleep loss, similarly decreased during the mating period.
While the decrease in oxalic acid suggests the agile antechinus were sleep deprived during mating season, the results show that the difference between males and females was not significantly different, which Zaid points out may suggest that females in the wild are similarly sleep deprived due to male harassment during the mating period.
“Our study is the first to compare male and female activity levels before, during, and after the breeding season, and to reliably relate restfulness with sleep using accelerometry, electrophysiology, and metabolomics,” researchers said in the paper.
Volker Sommer, a professor of evolutionary anthropology at University College London, said: “It rather seems that this is some pre-breeding stale-mate in not letting one’s guard down: males are forced to stay awake because their competitors also do.” Sommer was not involved in the study.
While the results don’t pinpoint a reason for the post-breeding male die-off, there are multiple possibilities, such as elevated corticosteroids — steroid hormones — and sleep deprivation.
Lesku said researchers would next like to examine how male antechinus deal with restricting their sleep for three weeks.
